1. Field of the Invention
The present invention relates to a novel diffraction grating and, more particularly, to a diffraction grating for being applied to an optical system.
2. Description of the Related Art
A spectrometer is typically implemented to measure photometry with regard to radiation sources, and a grating in such spectrometer is a component for diffusing a multi-frequency radiation. Instruments suchlike are extensively applied to deal with complex measurement tasks for acquiring accurate results. However, such instruments are currently disadvantageous by: (a) bulkiness resulted in great cost and using limitedly at fixed locations, (b) time consumption for wideband spectrum measurement, and (c) demand for skilled operators because cautious operation is necessary.
U.S. Pat. No. 5,550,375 provides an infrared-spectrometric sensor 100 for gases, as shown in FIG. 1, which comprises a microstructured body having a reflective grating 110, a multi-frequency IR radiation source 120, and a radiation receiver 130 for receiving IR of a fixed range of wavelength. Nevertheless, this infrared-spectrometric sensor is merely capable of measuring spectrums within a narrow wavelength range. In a case that multiple components are to be analyzed, the spectral signals would be absorbed at several different wavelengths, not only in the infrared region. Besides, the entrance slit, the two exit slits and the center of the mirror grating should be situated on a Rowland circle. Therefore, the applications of this prior spectrometric sensor are limited.
A simultaneous spectrometer 200 is another device for detecting radiation sources, as shown in FIG. 2. It comprises an entrance slit 200, a concave grating 210 capable of forming holographic images, and a photoelectric diode array 230. The aforementioned components are fixedly positioned and immovable while these components present the reliable advantages such as high accuracy and excellent optical efficiency. In such spectrometer, the photoelectric diode array is applied with limitations because the photoelectric diode array is substantially a flat plane formed with a large number of single crystals, while the focuses of the spectrometer are distributed on a curved surface and, more particularly, on the Rowland circle. In this case, only two intersection points could be formed between the Rowland circle and the photoelectric diode array. Thus, the spectral signals reflected by the concave grating 210 could be merely focused on the two intersection points of the photoelectric diode array. One preferred application of such simultaneous spectrometer is to increase the radius of the Rowland circle so that the distribution of the focuses can be a planar distribution approximately. However, this approach consumes large space and requires a large detector. An alternative solution is as the disclosure of U.S. Pat. No. 6,005,661, wherein a great quantity of optical fibers are employed to lead out signals with diverse wavelengths focused on the Rowland circle. Although such approach can compromise the disadvantages of photoelectric diode array, problems such as energy lost and degenerative resolution may also occur when the focused signals are led out by the optical fibers.
Instead, a diffraction grating generating linear outputs is a preferable option for an optical system. As shown in FIG. 3A, the inventor of U.S. Pat. Nos. 4,695,132 and 4,770,517 provides a laser scanning system 300, which implements one or more f θ lenses 310 to focus scattered light beams on a linear output plane 320. As shown in FIG. 3B, U.S. Pat. No. 6,650,413 provides a spectrometer 301 using a diffraction grating 311 and comprising an assembly of a collimator 313 and a correcting lens 315 for focusing the output spectral components on an image plane 321 in accordance with an f sin(θ) distribution.
However, the above-mentioned inventions are all systems with complex structures and therefore fail to achieve the objective of microminiaturizing an optical system to become portable.